Method of making coated paper



De. 5, 1944. E. G. BENNETT 2,364,505

METHOD OF MAKING COATED PAPER Filed July 13, 1940 INVENTQR fqyewe QBezmeZZ BY M I 244:. Q7/4401;

ATTORNEYS Patented Dec. 5, 1944 METHOD OF MAKING COATED PAPER Eugene G. Bennett, Hamilton, Ohio, assignor to The Champion Paper and Fibre Company, Hamilton, Ohio, a corporation of Ohio Application July 13, 1940, Serial No. 345,404

9 Claims.

This invention relates to a process of makingcoated paper, and in particular to an improved process employing formaldehyde and casein in the coating of paper with aqueous suspensions of mineral pigment and adhesive.

It has heretofore been proposed to use formaldehyde in paper coating compositions containing casein adhesives. Such coatings are relatively water resistant, and for this reason, have been suggested for washable wallpapers and like applications. Coated paper produced by such prior methods has relatively low resistance to penetration by printing ink and varnishes and for this reason, has not been found satisfactory in producing printing papers or papers capable of taking a varnish finish.

The primary object of the present invention is to provide a method of making coated paper which has an extremely high resistance to print ing ink and varnish penetration as compared to coated papers containing thesame ratios of pigment and casein made by previous methods, and for this reason, is suitable for gloss ink printing and for varnishing. Another object is to provide a method of making a varnishing grade of coated paper which is smoother and therefore has better printing properties than coated paper having equal printing ink penetration. Another object is to provide a method of coating paper which is economical because it results in the use of less casein than previously known methods.

I have discovered that by controlling the pH of the reaction medium at the time of the reaction between casein and formaldehyde in a layer of applied coating composition, I can make a coated paper of outstanding printing value. Ac cording to the invention, I bring about this reaction at a pH in excess of 9.0advantageously between 9.0 and 10.5, after the coating has been applied to the paper and while it is very wet.

Like certain coated papers heretofore proposed, which contained casein and formaldehyde, the papers of the present invention possess water resisting properties. between formaldehyde and casein is allowed to take place in the presence of fixed alkalies. at a controlled pH to produce the coated paper of However, when the reaction the present invention, the product acquires in addition an unusual degree of resistance to printing ink penetration not previously possessed by coated papers of the same casein content. The presence of controlled amounts of free fixed alkali which give this desired pH to the coating layer appears to promote the reaction between the casein and formaldehyde in some manner not clearly understood and produces a coated paper which has many of the desirable properties nor-,

mally associated with a coated paper having a higher casein content but without the undesirable properties of such a paper.

, The invention may be carried out in one of three general ways: (1) formaldehyde may be added to a layer of applied coating; or (2) suitable alkali may be added to a layer of applied coating containing, formaldehyde; or (3) a mixture of alkali and formaldehyde may be added to a layer of applied coating containing casein. The first modification is preferred.

According to' the preferred modification, the paper is first coated with an aqueous coating mixture containing mineral pigment and casein which has a pH of at least 9.0. In dissolving the casein for use in the coating mixture, sodium hydroxide, sodium carbonate, borax, sodium silicate or other alkaline sodium orpotasslum compounds may be used in such amount that after admixture with the pigment the coating composition has a pH of 9.0 or above. Alternatively an alkalin sodium compound may be added to the'mixture of casein solution and pigment to give the proper pH. The fixed alkalies are to be preferred over volatile alkalies like ammonia for giving the desired pH for the present purpose, because ammonia tends to combine with the formaldehyde more or less. After the coating has been applied to the paper and while the coating is still wet, the paper is'contacted with formaldehyde either as a gas or in solution. This operation may well be carried out by applying a solution of formaldehyde to the back of the paper by means of a roll dipping into a bath. A variation of this modification of the invention consists in coating a web of paper with an alkaline coating composition, drying and then superposing a second coating containing casein and formaldehyde, the first coating thus supplying the desired alkali for the more effective reaction between the casein and formaldehyde when the second coating is applied.

According to the second modification, there is first applied to the paper an aqueous coating mixture containing mineral pigment, casein and formaldehydeand then while the coating is still wet the freshly coated paper is contacted with a solution of an alkaline compound, as for example av dilute solution of sodium carbonate or calcium hydroxide. This may be done by passing the back of the paper over a roll which dips into the alkaline bath.

According to the third modification, the pag wet glossy appearance.

per is first coated with an aqueous coating mixture containing mineral pigment and casein, and either before or after the layer of coating has dried, it is brought into contact with a solution of formaldehyde containing a fixed alkali.

In any case, the casein and the formaldehyde react in alkaline medium advantageously shortly after the coating has been applied to the paper and while the coating is still very wet. The above noted particular method of bringing the reacting substances together-namely by wetting the back of the paper and thereby causing the reaction to proceed from the inner part of the coating outwards-is believed to be particularly effective for this purpose and is of course very simply carried out.

The invention will be described in connection with the accompanyin drawing which is a diagrammatic side view of a conventional brush coater to which has been added an apparatus suitable for carrying out the invention.

Referring to the drawing I is a web of paper which is shown being unwound from the roll of paper H. Ihe paper passes around guide rolls I2, I3 and Id. The coating is applied to the paper by means of a rotating brush l5 which picks up coating from the roll 5 which dips into the pan of coating l1. After the coating has been applied to the paper, the paper passes over a rubber apronl8 and the coating is brushed by the reciprocating brushes I9. is a solution-applying roll which rotates in a bath 2| of formaldehyde or alkaline compound as the case may be and applies an excess of the solution to the back of the paper. 22 is a small radius blade which bears against the back of the web and wipes off the surplus solution which has been applied and returns this excess to the pan 2|. 'After leaving the solution-applying device the paper passes to a dryer, not shown.

In order to obtain the most effective results with this process it is necessary to bring about the reaction between the casein, formaldehyde and alkali while the coating is very wet; that is to say, before the coating has soaked into the sheet sufficiently to have lost its characteristic If the formaldehyde is added to the coating while the paper is still damp but after it has lost its wet glossy appearance, the results, while good, are not as good as when it is added directly after the coating composition is applied. While there appears to be some value to the use of my process when applied to coating compositions of relatively low casein content, the results are not as striking as when used with coatings containing casein in excess of about 12% of the total dry weight of coating composition. Coated label paper, particularly of gloss ink and varnishing qualities, is commonly made with coatings containing casein in excess of 12%.

Paper coated according to the present invention possesses a much greater resistance to printing ink penetration than papers having the same ratio of casein to pigment but prepared according to previously known methods of uslng'formaldehyde in coatings. This property of the coated paper makes possible the manufacture of papers suitable for gloss ink printing and varnishing at lower cost and without the difficulties frequently encountered in the use of high casein content coatings. For example, in one coating formula formerly employed to give a gloss ink printing Paper, a ratio of 100 parts of special coating clay to 23 or 24 parts of casein was used. The coating composition contained formaldehyde. With such a high proportion of casein, foaming and spreading difficulties were usual and considerable waste was common, due to extreme curling of the paper in the drying lines. However, using the same pigment, a coated paper having the equivalent gloss ink printing properties was obtained by the present process with a 22% saving in casein. This paper was relatively free from pin holes, brush marks and objectionable curl and in addition, was smoother and glossier since it was easier to finish on the supercalenders due to the lower casein content.

The following example will illustrate the process. A web of paper such as is commonly used as a coating stock was coated on one side with a composition consisting on the dry basis of parts of coating clay, 25 parts finely precipitated calcium carbonate and 14 parts of casein. The casein was dissolved in water by the use of 25 parts of sodium carbonate for every parts of casein. The coating composition had a solids content of 38% and a pH of 9.5. The coating was appliedby means of a roll coater and within five seconds after the coating was applied to the web,

a 2% formaldehyde solution was applied to the back or uncoated side of the web by means of a roll dipping into a bath of the solution. Immediately after application of the formaldehyde the paper passed over a stationary scraper blade which wiped oil superfluous solution, which flowed back into the bath. The paper then passed to festoons where it was dried in the usual manner. It readily supercalendered to give a smooth, high grade varnish label paper. Its smooth, even surface made an excellent printing base and its high resistance to varnish penetration gave the paper excellent varnishing properties, so that the varnish remained on the surface with a high gloss and with substantially no discoloration. To obtain a paper of equivalent resistance to varnish penetration by other methods of coating using casein and formaldehyde, a 25% to 30% increase in casein would have been required, and printing results would have been inferior.

I claim:

1. Process of making coated paper which comprises applying an aqueous coating composition containing casein and pigment to paper and allowing the casein present in the moist coating layer on the paper to react with formaldehyde at a pH between 9.0 and 10.5.

2. Process of making coated paper which comprises applying to the paper an aqueous coating composition including casein and mineral pigment and having a pH between 9.0 and 10.5, and contacting the layer of applied, coating composition while still moist with formaldehyde.

3. Process of making coated paper which comprises applying to the paper an aqueous coating composition having a pH between 9.0 and 10.5 and including casein, mineral pigment and a nonvolatile alkaline compound, and while the layer of applied coating is still moist, applying formaldehyde to the back of the paper.

4. Process of making coated paper which comprises applying to paper an aqueous coating composition having a pH between 9.0 and 10.5 and including casein and mineral pigment, and while the layer of applied coating is still moist, apply- 7. Process of making coated paper which-comprises applying a layer of aqueous coating composition containing casein and mineral pigment to paper, thereafter bringing the coating in contact with formaldehyde while at a pH between 9.0 and 10.5.

8. Process of making coated paper which com-,-

, prises applying to paper an aqueous coating composition having a pH between 9.0 and 10.5 and containing mineral pigment, casein and an al kaline sodium compound, contacting theapplied coating with formaldehyde before the coating has lost its wet glossy appearance and drying the paper. p

9. Process of making coated paper of varnishing grade which comprises applying to a traveling web of paper an aqueous coating composition containing mineral pigment, casein in excess of 12% of the total dry weight of the coating composition, and sufficientnon-volatile a1- kaline compound to give a pH between 9.0 and 10.5, applying formaldehyde solution to the layer of coating While the coating still has a wet glossy appearance and drying the paper.

EUGENE G. BENNETT. 

